1. Field of the Invention
The present invention relates to an image forming machine for exposing a photosensitive recording medium to an image-forming light corresponding to image information to form a latent image of the image information on the surface of the photosensitive recording medium, and developing it to convert the latent image to a visible image. More particularly, the invention concerns an exposure apparatus which projects image-forming light for exposure using a plurality of light emitting elements and an image forming machine including the exposure apparatus.
2. Description of the Related Art
U.S. Pat. Nos. 4,440,846 and 4,399,209 disclose imaging systems in which a photosensitive layer having microcapsules containing a photosensitive substance as an internal phase is exposed image-wise to radiation, and subjected to a uniform rupturing force, whereby the microcapsules are ruptured to release the internal phase substance image-wise. According to these systems, the mechanical strength of the microcapsules is varied by exposure to form an exposed latent image. Under pressure, the capsules with a low mechanical strength (the capsules that have not been radiation cured, or the capsules that have been radiation softened) are destroyed to drain the inclusions such as coloring materials, which effects development.
These imaging systems are completely dry systems, and are markedly advantageous, because they do not rely on a wet developing solution to produce an image. An image forming coupler, such as a nearly colorless chromogenic material, is normally encapsulated in microcapsules. Once the microcapsules are ruptured, the chromogenic material (coupler) as a coloring material reacts with a developer material to form a colored image.
In the embodiments described in the quoted patents, microcapsules are normally ruptured by passing an image-wise exposed microcapsule coated sheet through the nip between a pair of cylindrical calender (polishing) rollers. Usually, high pressure, large polishing rollers are used to develop a microcapsule coated sheet. Even metallic polishing rollers with considerable precision are not smooth on the surface. When one of the rollers simply rests on the other roller, the surfaces of both rollers do not make contact over the entire width of the rollers. By applying a pressure to the rollers, their unsmooth surfaces o r the irregularities of their surfaces are smoothed, providing a uniform contact line between the two rollers. To achieve distribution of a microcapsule-destroying force throughout the surface of the microcapsule coated sheet, high pressure and large rollers are required. Unless the force is distributed evenly, the microcapsule coated sheet will not be developed uniformly, and the tone characteristics of the created image will be poor. As an example of developing system for resolving this disadvantage, Japanese Laid-Open Patent Publication No. 161153/87 discloses a point contact developing ball.
Known image forming machines for exposing such a photosensitive recording medium to image-forming light corresponding to image information, followed by developing it, include the image forming machine disclosed in Japanese Laid-Open Patent Publication No. 231758/87 corresponding to U.S. Pat. No. 4,740,809 which guides light from a white light source selectively to a photosensitive recording medium according to a print pattern; the system of Japanese Laid-Open Patent Publication No. 31364/88 corresponding to U.S. Pat. No. 4,740,809 which scans light from light sources of a plurality of colors and guides it to a photosensitive recording medium; the system of Japanese Laid-Open Patent Publication No. 369633/92 which guides light from a white light source selectively to a photosensitive recording medium through liquid crystal shutters having optical filters for a plurality of colors; and the system of U.S. Pat. No. 4,992,822 which exposes the same site in a photosensitive recording medium, capable of showing a plurality of colors, through a polygonal mirror a plurality of times.
U.S. Pat. No. 5,550,627 discloses an exposure and pressure applicator device for use in printing an image onto imaging media, containing photosensitive microcapsules. The device has an exposure means adjacent to the imaging media comprising at least one exposure-producing element which image-wise exposes the imaging media providing a latent image thereon; a pressure applicator means containing at least one point contact element to apply a uniform rupturing force to the exposed imaging media; and motive means for effectuating synchronous movement of both exposure means and pressure applicator means relative to the imaging media.
Among photosensitive recording media using no microcapsules are highly sensitive silver salt photographic media in which the necessary energy per unit area for exposure is less than about a hundredth of that required for photosensitive recording media using microcapsules. Known color image forming machines for exposing silver salt photographic media to image-forming light in accordance with color image information include image forming machines using color liquid crystal shutters, image forming machines using color CRT's, and image forming machines using color laser scan systems, as in the case of technologies for photosensitive recording media using microcapsules.
In recent years, there have been devised image forming machines using a plurality of light emitting elements to expose a photosensitive recording medium to image-forming light. With this type of image forming machine, a plurality of light emitting elements are fixed to an exposure head to be moved relatively along a photosensitive recording medium, and output light from the light emitting elements is selectively projected to the photosensitive recording medium. In detail, light is projected from the light emitting elements in all directions, so that only specified beams of the light (those beams perpendicular to the photosensitive recording medium) are used for exposure. To select such specified beams of light, a shield plate (mask) is provided which has pinholes formed opposite the light emitting elements.
With this type of image forming machine, however, only part of the total power of the output light from the light emitting elements is conveyed to the photosensitive recording medium through the shield plate in comparison with the energy per unit area necessary for the photosensitive recording medium. That is, only part of the energy can be used, making the efficiency of light utilization poor. Because of low sensitivity, the achievement of adequate exposure has taken time. For example, with respect to the total power of light from an LED (light emitting element), the rate of outgoing light through the corresponding pinhole (opening) is as low as about 10%. Most of the remaining light mingles with the equivalents in the path between the other LED's and the corresponding pinholes, and goes out through that pinhole as unnecessary background light which does not correspond to the image. As a result, deteriorations in image quality occur, such as worsened color reproduction of the image, poor lightness reproduction, and apparent decrease in resolution. In short, before output light from one LED appears through the corresponding pinhole, output light from other LED's mixes with it, with the result that untoward light exits through that pinhole, deteriorating the quality of the resulting image.
Furthermore, output light from the light emitting element is emitted in all directions on the surface of its mounting. If the light emitting elements are adjacent to each other, therefore, the problem arises that stray light from the adjacent light emitting element develops, exposing the photosensitive recording medium. Assume, for instance, that the pinhole, a, of the shield plate is opposed to the light emitting element, A, and the pinhole, b, of the shield plate is opposed to the light emitting element, B. Here, the stray light implicates that not only the output light from the light emitting element A passes through the pinhole a, but the output light from the light emitting element B also passes through the pinhole a after being reflected from the surface of the light emitting element A, the electrical connection, wiring, and so on. This means that if the stray light occurs when the light emitting element A is not on, there will be an exposed latent image formed when the light emitting element A has been lit.
Also imagine that the photosensitive recording medium is sensitive to light in a plurality of wavelength ranges, and that the light emitting element A emits light of the first wavelength, while the light emitting element B emits light of the second wavelength. In this case, should stray light develop, that part of the photosensitive recording medium which is opposite to the pinhole a may be exposed not only to light of the first wavelength, but to light of the second wavelength. This will make it difficult to form an exact exposed latent image. This is another problem that an unnecessary site is exposed to unintended light, debasing the quality of the image formed by the image forming machine.
The present invention has been aimed at solving the various problems with the conventional technologies. The object of the invention is to provide an exposure apparatus capable of increasing the efficiency of utilization of output light from light emitting elements, preventing the occurrence of stray light between a plurality of light emitting elements, and improving the quality of the resulting image; and an image forming machine including the exposure apparatus.